function [x_next, y_next] = f1(x, y, a, b, c, d)
% MNFM1 映射
%   x(k+1) = a*x(k)*(1 - x(k)) + b*sin(c*cos(y(k))*x(k)) + d
%   y(k+1) = y(k) + x(k)

    x_next = a * x * (1 - x) ...
           + b * sin(c * cos(y) * x) ...
           + d;
    
    y_next = y + x;
end